It is generally known that any mechanical testing machine has some eccentricity between its load cell and the cross-head. It is also known that the machining tolerances associated with the preparation of test bars often provides an imperfect geometry which can adversely affect the testing results. Most testing grips used to clamp specimens usually cannot allow any adjustment to correct the unavoidable misalignment due to the aforementioned factors. Despite this, the current trend in holding devices appears to be directed toward the facilitation of specimen installation through the use of hydraulic grips rather than the minimization of alignment errors. See in this regard MTS Systems Corporation, Grips and Fixtures Catalog, 300024-13/640.0305, MTS Systems Corporation, Box 24012, Minneapolis, Minn. 55424 and Trade Literature for the INSTRON Series 2742 and 2718, publications PS- 6-1088 and IC-6-66 (A), Instron Corporation, 100 Royall Street, Canton, Mass. 02021.
One popularly employed device, used often with single crystals where misalignment can cause unreliable measurements, is the WOOD ALLOY Grip System, whereby a set of alignment grips is connected to an alloy pot comprising molten metal. The grips are free to align with the specimen when the device is hot, but a cooling period of approximately 30 minutes to one hour is required prior to testing.
While this device has been popular, some testing problems have been associated with its use. With a heavy WOOD alloy pot on the top, the specimen can't really be freely aligned. The final alignment depends on the machining quality of the specimen grip part. Additionally, when connecting the WOOD ALLOY pot to a load cell of a mechanical testing machine, the cell will be heated up, resulting in considerable shifting of the load signal. Accordingly, an extended period of time is required to let the load cell cool down so that an accurate reading can be taken. Finally, the alignment of this device becomes unadjustable once the WOOD ALLOY solidifies.
Alternatively, at least one mechanical aligner has been introduced recently to overcome the heat-generated side effects caused by the WOOD ALLOY Grip. Z. Wang, Fatigue Behavior of 7030 Alpha Brass: Pauschinqer Effect and Surface Behavior in Single Crystals and High Cycle Fatigue Crack Formation at Twin Boundaries, Doctoral dissertation, Polytechnic Institute of New York, January, 1986, Microfilm recording at: University Microfilms, 30 M. Zeeb Road, Ann Arbor, Mich. 48106. This dissertation discloses a top grip body including a self-alignment hemisphere and a pair of specimen stoppers. The device also includes a bottom grip, configured similarly to the top grip, and including a hemispherical cavity. This device provides very little mobility to its flange portions, and thus, full employment of the mobility offered by the spherical center portion is impaired. In addition, torque may be created by the impingement of the flat portions of the bolts and nuts against the flanges which can render the reading unacceptable.
It is known that a misalignment of about 3.degree. from the axis of a tensile load on a half inch gage length specimen can produce unacceptable stress-strain information for objects such as soft, single crystal specimens. Moreover, a misalignment of only about 1.degree. off the axis of the tensile load can provide a misreading in the stress-strain curve of polymer specimens. Accordingly, there is a need for a mechanical testing alignment and gripping device that can provide for less than 1.degree. of misalignment. There is also a need for a faster alignment device that does not result in load cell heating with attendant inconvenience and reading inaccuracies.